Planetary surfaces

Abstract:

Ground test data in HDF5 and Matlab object formats from the ground testing of the Lunar Thermal Mapper instrument, 2023.

Abstract:

An accurate description of how visible light scatters from the lunar surface enables 1) constraints to be placed on the physical and compositional properties of the surface, using a photometric model such as the Hapke BRDF model, which has nine free parameters related to compositional and physical properties, and 2) more realistic scattering function inputs to be set within thermal models. Until a recent study by Foote et al. in 2010, lunar visible light scattering functions had been theoretically derived using limited laboratory measurements. Within thermal models, unrealistic scattering functions may be partly responsible for modelled temperature discrepancies of up to ~15-50 K (dependent on location)—when compared to remote sensing data from Diviner, onboard the Lunar Reconnaissance Orbiter—in regions such as polar craters, where light scattering due to surface topography dominates heat transfer. In this project, a laboratory goniometer setup was developed, which was used to measure a suite of visible light scattering functions for Apollo 11 (10084) and Apollo 16 (68810) lunar regolith samples across a wider range of viewing angles than has previously been measured. These samples were characterized in terms of their surface roughness and porosity profiles, and this enabled two of the free parameters within the Hapke BRDF model to be constrained. By fitting the model to the dataset, Hapke parameters could be deduced for the two representative (mare and highlands) regolith samples, and further constraints could be placed on the ‘practical’ size-scale of the model’s slope angle parameter. Thus, the dataset enabled Diviner’s visible-wavelength off-nadir data to be interpreted in a novel way, due to the reduction of free terms within the model. This led to surface roughness and compositional deductions (via the Hapke parameters h_s, b and θ ̅) for seven Diviner targets. Finally, the dataset was used to set more realistic scattering functions within the Oxford 3D Thermal Model, and it was demonstrated that this 1) could affect modelled high-latitude lunar surface temperature profiles by up to ~30 K—as compared to using previously assumed scattering functions—and 2) could increase the minimum depth at which water ice is predicted to be stable in the lunar subsurface by up to ~0.8 m. Hence, this dataset may help to constrain the possible distribution of water ice on the lunar surface, and this may be crucial for future lunar exploration missions such as Luna-27 and Artemis.

Abstract:

Ariel is an ESA mission that will use the transit spectroscopy method to observe the atmospheres’ of ~1000 exoplanets. Ariel is a 1 m class cryogenic space telescope that will be placed in a halo orbit around the Earth-Sun L2 point. To detect atmospheric molecular absorption features, Ariel will produce medium-resolution spectra (R ≥ 15) using three spectroscopic channels covering 1.1 – 7.9 µm as well as having photometric channels covering 0.5 – 1.1 µm. The technical driver for Ariel, though, is the photometric stability. This is to enable the detection of atmospheric spectral features that are a signal of tens of ppm relative to the host star. Ariel, therefore, aims to have tens of ppm stability over long (10 hr) timescales.

To achieve Ariel’s science goals, the payload requires detailed calibration and performance verification. The testing of the integrated Ariel payload will be the subject of this work. The ground calibration of the Ariel payload will take place in 2026 in the 5m vacuum chamber at the Rutherford Appleton Laboratory’s space instruments test facility. The payload will be enclosed in a Cryogenic Test Rig (CTR), to provide a space-like (35 K) thermal environment. During the cryogenic vacuum testing, the payload will be illuminated by the Optical Ground Support Equipment (OGSE). The OGSE used to calibrate the payload is being developed by a team led by Oxford University and the University of Lisbon.

The author joined the project just before mission adoption. The thesis was submitted just after the completion of the payload Preliminary Design Review (pPDR). The work completed in this thesis focused, therefore, on performance simulation, requirement derivation and design of the OGSE system. In this thesis, many of the key performance parameters will be derived. It will be shown how these parameters have shaped the OGSE system from an architectural level down to the detailed design.

To define the performance parameters of the OGSE, the calibration observation modes must be defined. Methods were, therefore, defined to show how the OGSE can be used to perform the payload-level calibration of Ariel. End-to-end radiometric simulations were also provided to simulate the focal plane signal when the payload is illuminated in the various calibration modes of the OGSE.

The photometric stability and dark current tests were identified as particular drivers for the OGSE design. The need for dark testing of the payload was used to derive the thermal requirements for the system. Finite Element Analysis (FEA) was then used to assess the thermal performance of the OGSE system, and thus enable low-background testing of the payload (<1 e-pix-1s-1 ). The other performance driver identified was the photometric stability test. End-to-end time-domain simulations of this test were performed to derive the required performance of OGSE flux monitoring systems. OGSE monitoring detector candidates were then assessed to demonstrate these could be used to verify the stability of the payload.

Alignment during ground testing was discovered to be a critical technical risk to the OGSE system. It will be shown how monitoring alignment from ambient to cryogenic temperatures led to a major redesign of the OGSE architecture. Moreover, Ariel’s tiny field of view (~30’’) leads to extreme (arcsecond) alignment maintenance requirements. An alignment monitoring system will be presented, built and verified, to enable the closed-loop monitoring required to keep the OGSE spot within the payloads’ spectrometer slits.